Mesh objects take the color of World (Blender 2.81a)
In renders, all objects reflect the World color.
White objects are affected most, they reflect it almost perfectly. Like in reality.

Solution 1 - Emission node
Use the Emission node for your object's color.
That way, the object will not receive any shadows or color tint.
It will just be a silhouette emiting white light.
Might work well for 2D animation.

Add > Shader > Emission



Comparing different materials.
Even colored materials get a slight tint. 



Solution 2 - Using the compositor

Available here on Blender Stack Exchange by the legendary cegaton himself.

Cegaton's answer offers a different approach using the compositor.
Rendered objects keep grey shading, but the background color can be changed at will.



Try what works best for you :).